The present invention relates generally to improvements in the acceleration resistance of quartz crystal resonators, and more particularly pertains to the elimination of acceleration-induced frequency shifts in such resonators.
Quartz crystals are commonly used to control the frequency of electrical oscillators and in other circuits where an electrical resonant frequency is required. A major problem with such crystals is that their natural resonant frequency changes when acceleration forces are applied to the crystal. The deleterious effects of these frequency shifts are well-known to designers of systems that require high-precision frequency control.
There are two classes of methods for reducing the acceleration sensitivity of a crystal: active and passive. In active methods, an acceleration sensor and a feedback network is used to alter the oscillator frequency and thereby compensate for the acceleration-induced frequency shifts. In passive methods, no attempt is made to sense the vibration or to dynamically change the output frequency.
One passive method of minimizing the acceleration-induced frequency shifts is disclosed in U.S. patent application Ser. No. 196,508, now U.S. Pat. No. 4,365,182. In that application, it is shown that acceleration resistant resonators can be produced from a single quartz plate that is optically twinned into a left-handed quartz portion and a right-handed quartz portion wherein the effective thickness of the two portions is equal. After depositing a pair of electrodes on each portion of the plate, the thickness of the electrode pairs is adjusted so that the resonant frequency of one portion is substantially the same as the resonant frequency of the other portion. The antiparallel alignment of the crystallographic axes accounts for the reduced sensitivity to acceleration.
Another approach to the problem is disclosed in U.S. patent application Ser. No. 086,504, now U.S. Pat. No. 4,344,010. That application shows that acceleration sensitivity can be minimized by the use of two separate resonators, one left-handed and one right-handed, that are aligned such that all three crystallographic axes are antiparallel.
Although these earlier methods minimize the acceleration-induced frequency shifts, none of the methods completely cancels the effects of acceleration forces due to the fact that all of the approaches consist of aligning only the crystallographic axes to eliminate the effects of vibration.